(a) Field of the Invention
The present invention relates to an image display and a display panel thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display.
(b) Description of the Related Art
In general, an organic EL display is a display that emits light by electrical excitation of fluorescent organic compound and displays images by driving each of N×M organic luminescent cells with voltages or current. The organic cell has a structure of an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film is formed as a multi-layered structure including an emission layer (“EML”), an electron transport layer (“ETL”) and a hole transport layer (“HTL”) so as to increase luminescence efficiency by balancing electron and hole concentrations. In addition, it can include an electron injection layer (“EIL”) and a hole injection layer (“HIL”) separately.
Organic EL displays that use organic luminescent cells such as the above are configured as a passive matrix or an active matrix that includes thin film transistors (TFTs). In the passive matrix configuration, organic luminescent cells are formed between anode and cathode lines that cross each other and are driven by driving those lines. In the active matrix configuration, a TFT and a capacitor are coupled to each ITO pixel electrode to maintain voltage using capacitance. Here, the active matrix type organic EL display displays images according to a voltage programming method and a current programming method that are dependent on the form of a signal applied to maintain the voltage in the capacitor.
In the voltage programming method, data voltages representing gray scales are supplied to a pixel circuit to display an image. This voltage programming method has a problem of unevenness due to a variation in threshold voltages and electron mobilities of driving transistors. In the current programming method, data current representing gray scales is provided to the pixel circuit to display an image. The current programming method can solve the unevenness problem. However, the current programming cannot properly secure a charging time for charging loads of data lines because it should control organic EL devices with a small quantity of current.
An example of the pixel circuit for compensating a threshold voltage of a driving transistor in the voltage programming method is disclosed in U.S. Pat. No. 6,362,798 by Mutsumi et al. This pixel circuit compensates the threshold voltage of the driving transistor using a mirror transistor whose gate is coupled to the gate of the driving transistor.
In the image display having the pixel circuit that compensates the threshold voltage of the driving transistor using the mirror transistor, i.e., the pixel circuit using two mirror-type transistors, the transistors must have the same characteristics. In a conventional method of manufacturing such image display device, an Eximer laser (ELA) is used. FIG. 1 shows the traveling direction of an Eximer laser used to manufacture the conventional image display device.
For the case where R, G, and B pixels are arranged on the same line to form a pixel, as shown in FIG. 1, two mirror-type transistors are arranged in such a manner that the channel directions of the two transistors are parallel with the traveling direction of the Eximer laser in each pixel. Further, the Eximer laser is irradiated on each pixel with its traveling direction being parallel with a longer side of each pixel. Here, the same Eximer laser beam is not irradiated on the R, G, and B pixels, but different Eximer laser beams are irradiated on them as shown in FIG. 1. This makes the driving transistors of the R, G, and B pixels have different characteristics, thereby resulting in unevenness of luminance.